1. Field of the Invention
This invention relates to an injection molding method using an injection mold having a gate, and also to an injection mold for use therewith.
2. Description of the Related Art
FIG. 4 shows a conventional injection molding machine. In such an injection molding machine, two injection devices 50 and 52 are provided upstream of an injection nozzle 56, and a rotary valve 54 is provided in a resin passage between the injection nozzle 56 and the injection devices 50 and 52. The injection nozzle 56 is pressed against a mold 58. By switching the angular position of the rotary valve 54 so as to select one of the two injection devices 50 and 52, proper injection can be effected. For example, a skin layer resin material is first injected from the injection device 50 into a cavity C, and then the rotary valve 54 is switched, and a core layer resin material is injected from the injection device 52 into the cavity C. By doing so, a so-called sandwich molded product 64 having a core layer 62 covered with a skin layer 60 can be produced. FIG. 4 shows the condition of injecting of the core layer resin material from the injection device 52.
However, this type of machine has disadvantages. In particular, it is difficult to mold a sandwich molded product having a smooth surface. This is so because, during the time when the injection is switched from the skin layer resin injecting condition to the core layer resin injecting condition, there is a time period during which no resin is injected into the cavity C. Because of this temporary stoppage in the flow of the molten resin, a so-called hesitation mark is formed on the surface of the sandwich molded product. This detracts from the appearance of the final product. The above machine also has a disadvantage in that it is difficult to effect a multi-shot molding.